The present invention relates to medical catheters or probes which are placed within living tissue relative to the vascular or neurological anatomy of a patient (which may be a human or an animal) to perform a measurement, sensing or stimulation function. More particularly, the invention provides a system for introducing a select fluid, e.g., a liquid chemical, into the environment of the functional end of the catheter or probe. The liquid chemical may be selected to prevent, or reduce, formation of thrombus, tissue growth and encapsulation of the invasive portion of the catheter, to medicate and prevent infection at the site of implantation, to anesthetize surrounding neurostructure or to apply a calibration or control substance into the investigated environment.
Catheters and probes are utilized for numerous measurement, sensing, and stimulation functions of body fluids and tissues, typically through an invasive procedure in which the catheter or probe is inserted within a vascular member, e.g., a vein or artery, through which the investigated fluid flows or implanted within the investigated tissue. The catheter or probe usually is constructed to form a lumen through which a sensor or electrode and its conducting element is passed to the site of interest and terminates with the sensor or electrode protruding from its end to gain exposure to the investigated fluid or tissue. For example, an in vivo sensor for measuring pH, glucose or oxygen content in blood may be delivered to the select site through a catheter inserted in an artery leading to the site. Alternatively, an electrode for stimulating muscle tissue may be brought to the site of excitation through a tubulus probe implanted in adjacent bodily tissue.
While the catheter or probe is constructed of a biocompatible material, protective mechanisms of the body generally operate on the invasive device to protect the body from the invasion of the foreign object. The most common protective mechanism of the body is encapsulation of the foreign object by a growth of isolating tissue. Obviously, the encapsulation of the end of the catheter or probe from which the sensor or electrode protrudes will significantly reduce or cease desired function of the in vivo device. Further, and in particular with regard to a catheter placed within a blood vessel, blood clotting may occur at the end of the catheter where a sensor extends or is exposed resulting in reduced effectiveness of the in vivo device, as well as blockage of the vessel by the clot. This is clearly an undesirable result.
Antithrombogenic chemical substances are known which reduce and slow the formation of thrombus, tissue growth, blood clotting and the encapsulation mechanism. For example, the substance known as "heparin" is a known anticoagulant which inhibits the action of the enzyme thrombin in the final stage of block coagulation. Heparin is generally applied in dilution in saline solution. The use of such substances with catheters and probes to reduce the undesirable effects of the body's protective mechanisms has been taught. For example, U.S. Pat. No. 4,954,129, issued to Giuliani, et al., teaches the use of theses chemical substances for hydraulic flushing of thrombus or clotting material from within the lumen of an intravascular catheter having an open end from which a sensor probe extends. The Giuliani et al. disclosure further provides a defined process for flushing the bore of the catheter which utilizes a periodically increased velocity pulse of a flushant to remove incipient clot material formations. A specific structure is described and claimed for centering a sensor probe within the lumen of the catheter and away from the catheter wall which provides fluid channels for the flow of the flushant through and from the end of the catheter around the probe sensor.
Another U.S. Pat. No. 4,934,369, issued to Maxwell, deals with the subject problems in a similar manner. While Maxwell teaches the technique of flowing antithrombogenic fluid through and out of the end of the intravascular catheter around the sensors for measuring blood constituents, he also teaches the positioning of the sensors within the catheter lumen a select distance from the opening at the end of the catheter. Blood is permitted to enter the end of the catheter and/or through ports about the circumference of the catheter and mix with the antithrombogenic fluid to forming an interface zone therebetween. The blood-fluid interface zone is washed back and forth over the sensors to expose the sensors to both blood and antithrombogenic fluid by one of various described means for pulsing the flow of fluid within the catheter lumen.
While these references appear to disclose functional systems for applying an antithrombogenic fluid to the environment of a sensor, both concern catheters which have an opening at their distal end which permit the sensor exposure to blood, either by the sensor extending through the opening to come into contact with blood, or by blood entering the end opening to contact the sensor immediately adjacent therewith. In both constructions the antithrombogenic fluid flows through the lumen of the catheter and exits the end of the catheter, requiring some means to position the sensor centrally within the end opening in order for it to obtain uniform exposure and for uniform fluid infusion. Such configurations require constructions of complex design which, of course, substantially increase cost. The Maxwell design, for example, is subject to sensitive dynamic control to assure proper and effective association of the sensor with blood entering the end opening of the catheter. The Giuliani et al. design consists of tortuous surface shapes in the bore and adjacent to the end of the catheter, all of which provide fertile areas for the incipiency of thrombus or clot formation, in direct opposition to the intended purpose. Tooling used during manufacture is complex and expensive. Further, with the Giuliani et al. design, sensor positioning within the catheter and in relation to the end opening becomes a critical parameter for effective operation, which proper positioning is difficult to ascertain once the catheter has been inserted into a patient's blood vessel.
It is thus apparent that improvements are needed in the way in which select fluids or liquid chemicals are introduced to an in vivo sensing or stimulating location for the purpose of reducing the undesirable effects of the body's protective mechanisms.